The invention relates generally to an electrophysiological (xe2x80x9cEPxe2x80x9d) system and method for providing energy to biological tissue within a biological site, and more particularly, to an EP system and method for controlling the delivery of RF energy to the tissue based on the flow of fluid through the biological site.
The heart beat in a healthy human is controlled by the sinoatrial node (xe2x80x9cS-A nodexe2x80x9d) located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node (xe2x80x9cA-V nodexe2x80x9d) which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as xe2x80x9ccardiac arrhythmia.xe2x80x9d
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed by percutaneous ablation, a procedure in which a catheter is percutaneously introduced into the patient and directed through an artery to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue.
In the case of atrial fibrillation (xe2x80x9cAF xe2x80x9d), a procedure published by Cox et al. and known as the xe2x80x9cMaze procedurexe2x80x9d involves continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system providing RF ablation therapy. In this therapy, transmural ablation lesions are formed in the atria to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium.
During ablation, electrodes carried by an EP catheter are placed in intimate contact with the target endocardial tissue. RF energy is applied to the electrodes to raise the temperature of the target tissue to a non-viable state. In general, the temperature boundary between viable and non-viable tissue is approximately 48xc2x0 Centigrade. Tissue heated to a temperature above 48xc2x0 C. becomes non-viable and defines the ablation volume. The objective is to elevate the tissue temperature, which is generally at 37xc2x0 C., fairly uniformly to an ablation temperature above 48xc2x0 C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100xc2x0 C. When the blood temperature reaches approximately 100xc2x0 C., coagulum generally occurs.
Blood coagulation is a major limitation/complication associated with RF ablation therapy. Coagulation can lead to thromboembolism and also form an insulating layer around the electrode hindering further energy delivery required for ablation therapy. Thus, heating of blood is a major concern for ablation safety and efficacy. During ablation therapy, it is known that the temperature of blood near an electrode is dependent on the blood flow rate. Low blood flow results in reduced convective heat dissipation within the blood pool around the electrode and thus higher blood temperature. Conversely, high blood flow rate results in increased convective heat dissipation within the blood pool around the electrode and thus a lower blood temperature.
Conventional RF ablation systems fail to account for the effect that varying blood flow rates have on blood, electrode and tissue temperatures, which can be substantial. During an ablation procedure, conventional systems apply a level of RF energy to the electrodes sufficient to elevate the tissue temperature to a level that causes the tissue to become non-viable. The level of RF energy is generally constant regardless of the blood flow rate and is only adjusted if the system employs some type of temperature feedback control. In these systems an attempt is made to guard against blood coagulation and coagulum formation by monitoring the temperature of the electrodes, usually using a thermocouple attached to the electrode. When a threshold temperature is reached, the application of RF energy is either reduced or shut off. However, such thermocouples are generally located at the tissue/electrode contact location, which can have a significantly different temperature than the opposite side of the electrode that is in the blood pool.
Such systems tend either to have a high incidence of coagulation or to operate inefficiently. Coagulation is likely to occur in these systems when the RF energy delivered to the electrode is set to an ablation-inducing level during periods of high-blood flow. The temperature sensing thermocouple does not provide the system with sufficient information about the temperature of the blood pool. Consequently, the convective heat dissipation effect of the high-blood flow keeps the blood pool around the electrode cool and ablation is efficiently accomplished, however, during periods of low-blood flow, the reduced convective heat dissipation allows the blood pool to heat. Over the course of an ablation procedure, the cumulative effect of the periods of low-blood flow is likely to result in coagulum formation.
In order to avoid coagulation the energy level may be reduced. This, however, tends to lead to an inefficient ablation procedure. If the energy level is set to induce ablation during periods of low flow, the convective heat dissipation effect during periods of high-blood flow reduces the electrode temperature and thus the tissue temperature to a non-ablative level. The culmination of these periods of non-ablative temperature levels at best increases the amount of time necessary to achieve an ablation-inducing temperature and thus the overall procedure time, and at worst prevents the electrode from ever reaching an ablation-inducing level.
Hence, those skilled in the art have recognized a need for a RF ablation system and method that controls and adjusts the RF energy level delivered to tissue within a biological site based on the flow rate of fluid through the site. The invention fulfills this need and others.
Briefly and in general terms, the present invention is directed to a system for, and a method of, controlling the delivery of energy to biological tissue during an ablation procedure based on the flow of fluid through the biological site.
In one aspect, the invention relates to a system for applying energy to biological tissue within a biological organ having fluid flowing therethrough. The system includes a generator for providing energy and a catheter carrying an electrode system at its distal end. The distal end of the catheter is adapted to be positioned in a biological organ and the electrode system is adapted to receive energy from the generator. The system further includes a device adapted to provide flow rate information indicative of the flow rate of the fluid through the biological organ and a processor adapted to receive the flow rate information. The processor is adapted to assess whether the fluid-flow rate is high or low and control the generator such that the generator provides energy of a first level to the electrode during periods of high fluid-flow and energy of a second level, less than the first level, during periods of low fluid-flow.
By providing a processor that controls a generator such that energy of a first level is applied during periods of high fluid-flow and energy of a second level, less than the first level is applied during periods of low fluid-flow, the system is able to dynamically control the application of energy to the electrode based on the rate of fluid flow through the biological site. In linking the applied energy level to the fluid-flow rate, the system accounts for the effect that varying fluid-flow rates have on blood, electrode and tissue temperatures and maintains the electrode at or near an ablative temperature throughout an ablation procedure, thereby increasing ablation efficiency and reducing chances of blood coagulation.
In a detailed aspect of the invention, the processor controls the generator to increase the energy level to the first energy level at the beginning of the high flow period and to decrease the energy level to the second energy level toward the end of the high flow period and before the beginning of the next low flow period. In another detailed facet, the system further includes a temperature sensor for providing temperature signals to the processor. The signals are indicative of the temperature at the electrode system. In this facet, the processor is adapted to determine the temperature at the electrode system based on the temperature signals and to control the generator such that the level of energy applied to the electrode system maintains the temperature of the electrode system at or near a target temperature. In a further detailed aspect, the processor is adapted to adjust the level of energy output by the generator during a subsequent low/high period based on the temperature of the electrode system during the previous low/high period. In another detailed aspect, the system further includes a temperature sensor for providing temperature signals to the processor. The signals are indicative of the temperature at the electrode system. In this aspect, the processor is adapted to determine the temperature at the electrode system and to control the generator such that the level of energy applied to the electrode system maintains the temperature of the electrode system below a maximum threshold temperature.
In another detailed aspect of the invention, the flow rate information device includes an electrocardiograph (ECG) device adapted to monitor changes in electrical activity and the flow rate information includes ECG signals. In a further detailed aspect, the ECG device includes either one or both of an internal ECG sensor and an external ECG sensor. In another further detailed aspect, the ECG device includes at least one ECG filter in electrical communication with the electrode system. The ECG filter receives electrical signals from the electrode system and outputs them as ECG signals. For an ECG signal providing a waveform having a sequence of alternating P waves and T waves, the processor is adapted to identify the periods between a P wave and its subsequent T wave as high fluid-flow periods and the periods between a T wave and the next P wave as low fluid-flow periods. For an ECG signal providing a waveform having a sequence of alternating QRS complex waves and T waves, the processor is adapted to identify the periods between the onset of a QRS complex wave and the subsequent T wave as high fluid-flow periods and the periods between a T wave and the next QRS complex wave as low flow periods.
In another detailed aspect of the invention, the flow rate information device includes at least one flow sensor located near the electrode system that is adapted to sense fluid flow and the flow rate information includes velocity values. In a further detailed aspect, the processor is adapted to identify periods during which the sensor signals provide a velocity value greater than or equal to a predetermined velocity value as high fluid-flow periods and those periods during which the velocity value is less than the predetermined velocity value as low fluid-flow periods.
In another aspect, the invention relates to a method of applying energy to biological tissue within a biological organ. The method includes the steps of positioning an electrode within the biological organ, such that a portion of the electrode contacts the biological tissue and determining the biological fluid-flow rate within the biological organ. The method further includes the steps of, during periods of high biological fluid flow, applying energy of a first level to the biological tissue and during periods of low biological fluid flow, reducing the level of energy applied to a second level, less than the first level.
In a detailed aspect of the invention, the first level of energy is substantially constant and the second level of energy is substantially zero. In another detailed aspect, the steps of applying energy of a first level and reducing the level of energy to a second level includes the steps of increasing the energy to the first energy level at the beginning of a high flow period and decreasing the energy level to the second energy level toward the end of the high flow period and before the low flow period.
In another detailed aspect of the invention, the method further includes the steps of monitoring the temperature of the electrode and adjusting the level of energy applied to the electrode to maintain the temperature of the electrode at or near a target temperature. In a further detailed facet, the method further includes the step of, for a sequence of alternating high flow rate periods and low flow rate periods, adjusting the level of energy during a subsequent low/high period based on the temperature of the electrode during the previous low/high period.
In another detailed facet of the invention, the step of determining the biological fluid-flow rate includes the steps of measuring changes in voltage occurring in the human body with each heart beat to produce an electrocardiogram waveform having a sequence of alternating P waves and T waves and identifying high fluid-flow periods as those periods between a P wave and its subsequent T wave and low fluid-flow periods are those periods between a T wave and the next P wave. In another detailed aspect, the step of determining the biological fluid-flow rate includes the steps of measuring changes in voltage occurring in the human body with each heart beat to produce an electrocardiogram waveform having a sequence of alternating QRS complex waves and T waves and identifying high fluid-flow periods as those periods between the onset of a QRS complex wave and the subsequent T wave and low fluid-flow periods are those periods between a T wave and the next QRS complex wave. In yet another detailed aspect, the step of determining the biological fluid-flow rate includes the steps of measuring the velocity of the fluid flow and identifying those periods during which the sensor signals provide a velocity value greater than or equal to a predetermined velocity value as high fluid-flow periods and those periods during which the velocity value is less than the predetermined velocity value as low fluid-flow periods.
In another aspect, the invention relates to a system for applying energy to biological tissue within a biological organ having fluid flowing therethrough. The system includes a generator for providing energy and a catheter carrying an electrode system at its distal end. The distal end is adapted to be positioned in a biological organ and the electrode system is adapted to receive energy from the generator. The system further includes a device adapted to provide flow rate information indicative of the flow rate of the fluid through the biological organ and a processor adapted to control the generator such that the generator provides energy to the electrode system based on the flow rate information.
In a detailed aspect of the invention, a preset flow rate and a maximum energy level are programmed into the processor and the processor is adapted to compare the measured flow rate to the preset flow rate. When the measured flow rate is greater than or equal to the preset flow rate, the processor sets the provided energy level to the maximum energy level. When the measured flow rate is less than the preset flow rate, the processor determines the rate of reduction of the measured flow rate relative to the preset flow rate and sets the provided energy level to a value less than the maximum energy level. The provided level is a multiple of the maximum energy level and the multiple is set based on the determined reduction rate.
In another detailed aspect of the invention, a preset flow rate, a high target temperature, and a low target temperature are programmed into the processor. The processor is adapted to monitor the temperature of the electrode and compare the measured flow rate to the preset flow rate. When the measured flow rate is greater than or equal to the preset flow rate, the processor determines the rate of increase of the measured flow rate relative to the preset flow rate and sets the applied energy level to a value greater than the current energy level, the applied level being a multiple of the current energy level and the multiple being set based on the determined increase rate. The processor also compares the electrode temperature to the high target temperature and adjusts the applied energy level to maintain the electrode temperature near the high target temperature. When the measured flow rate is less than or equal to the preset flow rate, the processor determines the rate of reduction of the measured flow rate relative to the preset flow rate and sets the applied energy level to a value less than the current energy level, the applied level being a multiple of the current energy level, the multiple being set based on the determined reduction rate. The processor also compares the electrode temperature to the low target temperature and adjusts the applied energy level to maintain the electrode temperature near the low target temperature.
In another facet, the invention relates to a method of ablating biological tissue within a biological organ having biological fluid flowing therethrough. The method includes the step of positioning an electrode within the biological organ such that a portion of the electrode contacts the biological tissue. The method further includes the steps of measuring the biological fluid-flow rate within the biological organ and applying energy to the electrode based on the flow rate measurement.
In a detailed aspect of the invention, the step of measuring the biological fluid-flow rate includes the steps of positioning a flow sensor within the biological fluid and determining the flow rate of the biological fluid. In a further detailed facet, the step of applying energy to the electrode based on the flow rate measurement includes the steps of establishing a preset flow rate and a maximum energy level and comparing the measured flow rate to the preset flow rate. The method further includes the step of, when the measured flow rate is greater than or equal to the preset flow rate, setting the applied energy level to the maximum energy level. When the measured flow rate is less than the preset flow rate, the method includes the steps of determining the rate of reduction of the measured flow rate relative to the preset flow rate and setting the applied energy level to a value less than the maximum energy level, the applied level being a multiple of the maximum energy level, the multiple being set based on the determined reduction rate.
In another detailed aspect of the invention, the method further includes the steps of monitoring the temperature of the electrode and adjusting the level of energy applied to the electrode to maintain the temperature of the electrode at or near a target temperature. In a further detailed aspect, the steps of applying energy to the electrode based on the flow rate measurement and adjusting the level of energy applied to the electrode to maintain the temperature of the electrode at or near a target temperature includes the steps of establishing a high target temperature, a low target temperature and a preset flow rate value, monitoring the temperature of the electrode and comparing the measured flow rate to the preset flow rate. When the measured flow rate is greater than or equal to the preset flow rate, the method includes the further steps of determining the rate of increase of the measured flow rate, setting the applied energy level to a value greater than the current energy level, the applied level being a multiple of the current energy level, the multiple being set based on the determined increase rate. Also included are the steps of comparing the electrode temperature to the high target temperature and adjusting the applied energy level to maintain the electrode temperature near the high target temperature. When the measured flow rate is less than or equal to the preset flow rate, the method includes the further steps of determining the rate of reduction of the measured flow rate, setting the applied energy level to a value less than the current energy level, the applied level being a multiple of the current energy level, the multiple being set based on the determined reduction rate. Also included are the steps of comparing the electrode temperature to the low target temperature and adjusting the applied energy level to maintain the electrode temperature near the low target temperature.
In another aspect, the invention relates to a system for applying energy to biological tissue within a biological subject having fluid flowing therethrough. The system includes a generator for providing energy and a catheter carrying an electrode system. The catheter is adapted to be positioned in a biological organ and the electrode system adapted to receive energy from the generator. The system further includes a device adapted to provide flow rate information indicative of the flow rate of the fluid through the biological subject and a processor adapted to control the generator such that the generator provides energy to the electrode system based on the flow rate information.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings which illustrate by way of example the features of the invention.